1. Field
The present disclosure relates generally to apparatus and methods for spectrum sharing using Listen-Before-Talk (LBT) and quiet periods, and more specifically to spectrum sharing among heterogeneous systems using LBT and quiet periods with an accounting for time of transmission.
2. Background
The proliferation of wireless devices and applications has generated a huge demand for bandwidth that is expected to grow well into the future. Unfortunately, bandwidth is very scarce and hence an efficient utilization of this resource is crucial. The Federal Communication Commission's (FCC's) established frequency allocation shows a heavily crowded and fragmented spectrum with most frequency bands already assigned to different licensed (primary) users for specific services. However, studies conducted by the FCC and in the industry show that on the average only about 15% of the spectrum is used at any given time and location.
Accordingly, several techniques for sharing spectrum among different users, such as those that can be encompassed by the term “cognitive radio,” have been developed that seek to overcome the spectral shortage problem by enabling secondary (unlicensed) wireless devices to communicate without interfering with the primary (licensed) users of a frequency bands. Cognitive radio communication faces a multitude of challenges in spectrum access, coexistence and sharing. In single secondary user environments, the spectrum access problem reduces to that of primary user sensing. Based on the sensing metric and the interference tolerable at the primary users, the secondary user decides to either transmit (when no primary user is detected) or goes to an idle state (when a primary user is detected).
With multiple secondary users in the same system, however, more complex coexistence protocols are required to ensure efficient sharing of the spectrum. Within the same wireless network, sharing among secondary users reduces to a conventional multiple access problem that can be controlled by the base station (BS). Consequently, many access protocols that have been proposed for spectrum sharing among homogeneous secondary users are derived from conventional medium access control (MAC) protocols like ALOHA and carrier sense multiple access (CSMA).
When there are multiple, non-interoperable networks sharing a spectrum, this is referred to as spectrum sharing between heterogeneous wireless networks. In the case of heterogeneous networks differing in the main system parameters (e.g., radio coverage, transmit power, data rates, channelization, or media access protocols), spectrum sharing becomes especially complex. The main challenge to opportunistic communication in such diverse environments lies in striking a balance between the conflicting goals of minimizing the interference among the secondary users and at the same time maximizing the performance that each system can achieve under some fairness conditions, such as LBT.
In order for two heterogeneous frame-based systems, such as OFDMA systems, to coexist, however, LBT is not sufficient. This is because if a secondary system is already transmitting with a full buffer, for example, then any other secondary user will be blocked because the medium will always be sensed busy. Therefore, for frame-based systems a quiet period (QP) is scheduled to give opportunity for other systems to acquire the medium. As one known example, use of LBT along with extended quiet period (EQP) protocols was proposed in an IEEE 802.16h draft for the coexistence of IEEE 802.16h with other secondary users (i.e., “Improved Coexistence Mechanisms for License-Exempt Operation,” IEEE P802.16h/D2c, July 2007). In the extended quiet period (EQP) protocol, the system alternates deterministically between active and silence periods to satisfy a specific duty-cycle defined for the system. For example, if the system has a duty-cycle of 50% then the system can transmit every other frame; one frame on and one frame off.
LBT was also proposed, for example, as an independent protocol of the extended quiet period, and both the base station (BS) and the mobile station (MS) are supposed to apply it. In a later version of the IEEE 802.16h draft (i.e., Improved Coexistence Mechanisms for License-Exempt Operation, IEEE P802.16h/D3c, October 2007) the LBT protocol was changed to coexist specifically with IEEE 802.11 wireless local area networks. In particular, LBT was proposed to be performed at the BS only and the BS is equipped with an IEEE 802.11 transceiver that has the capabilities of sending and receiving IEEE 802.11 CTS/RTS signals. Because of this, the later draft states that the use of LBT and quiet period are optional. In both drafts of 802.16h discussed above, if both LBT and the extended quiet period are used simultaneously and if a frame during the active period is not transmitted because of the LBT protocol, then this idle frame will count toward satisfying the extended quiet period duty-cycle, which can lead to inefficient use of the spectrum. Furthermore, the conventional art does not specify how the LBT and quiet period algorithms should be combined or how a duty cycle target should be satisfied. Accordingly, there exists a need for improved efficiency of spectrum sharing among frame based networks using LBT and extended quiet period.